Assessing seepage sources of a tailings dump and fractionation of Mo and Zn isotopes.

Tracing the sources of each contaminant and its geochemical reactions requires a variety of geochemical tools. In this study, chemical compositions and isotopic ratios of O-H, Mo, and Zn were utilized to identify the sources and geochemical reactions of water, As, Mo, and Zn in the seepage from a mine tailings dump. The distinct chemical compositions observed between the seepage and monitoring well, along with the O-H isotopic ratios, suggested that the seepage originated from creek water rather than nearby groundwater, which was supported by a large seasonal variation of δMo in both the seepage and creek. Interpretation results indicated that Mo was predominantly supplied from the creek, while the majority of As and Zn originated from the tailings. During the transport of Mo and Zn, δMo and δZn increased and decreased, respectively, suggesting adsorptive removal, despite the δZn increase during the leaching of the tailings. Notably, the combined use of Mo and Zn isotopic ratios proved to be a valuable tool for identifying geochemical reactions and determining sources and pathways in complex environmental systems. Additionally, although As does not have multiple isotopes, possible adsorption of As onto Fe (oxy)hydroxides could be inferred based on the isotopic behavior of Mo and Zn, as these two isotopes effectively reflected isotopic fractionation during adsorption.